This makes pkexec(1) work when e.g. logging in via ssh(1) or the linux
console but also when using `su -'. Example:

 [davidz@x61 ~]$ su - bateman
 Password:
 [bateman@x61 ~]$ pkexec bash
 ==== AUTHENTICATING FOR org.freedesktop.policykit.exec ===
 Authentication is needed to run `/bin/bash' as the super user
 Authenticating as: root
 Password:
 ==== AUTHENTICATION COMPLETE ===
 [root@x61 ~]#

Summary of changes

 - Added a PolkitAgentTextListener class

 - Add new polkit_agent_listener_register() (and _unregister()) API

 - Deprecate polkit_agent_register_listener API

 - Allow registering authentication agents for PolkitUnixProcess subjects
   and prefer such agents to ones governing the session

 - Make PolkitAgentSession use the thread-default GMainContext - otherwise
   it won't work in spawned threads

 - (finally) use PolkitAgentTextListener in pkexec(1) if authorization
   via authentication is possible but no authentication agent was
   found

Signed-off-by: David Zeuthen <davidz@redhat.com>